1. Field of the Invention
The present invention relates to an aluminum alloy substrate for an electrolytically grainable lithographic printing plate having a good electrolytic graining property, particularly excellent in uniformity of appearance after electrolytic graining.
2. Description of the Related Art
Conventional aluminum alloy substrates for a support for an electrolytically grainable lithographic printing plate are generally provided in the form of an about 0.1 to 0.5 mm thick cold-rolled sheet made of an aluminum alloy such as JIS A1050, A1100, A3003, or the like. Such aluminum alloy cold-rolled sheets are generally produced by machining the surface of a semicontinuous-cast (DC) slab, homogenization heat-treating the slab when necessary, heating the slab to a selected temperature, hot-rolling the heated slab to a hot-rolled strip, cold-rolling the hot-rolled strip with an intermediate annealing between the cold rolling passes, and final cold rolling the strip to a cold-rolled sheet.
Japanese Unexamined Patent Publication (Kokai) No. 3-79798 discloses a process for producing an aluminum alloy support for an electrolytically grainable lithographic printing plate, in which an aluminum alloy melt is continuously cast and rolled to form a strip coil which is then subjected to cold rolling, heat treatment, and straightening.
Japanese Unexamined Patent Publication (Kokai) No. 5-156414 discloses a process for producing an aluminum alloy support for an electrolytically grainable lithographic printing plate, which comprises carrying out twin-roll continuous casting and rolling and then hot rolling to prepare a strip coil having a thickness of 4 to 30 mm which is then cold-rolled, with heat treatment at a temperature of 400.degree. C. or above being carried out in the course of the cold rolling when the thickness of the rolled sheet has reached 1 mm, and further cold-rolled. It further discloses a process for producing an aluminum alloy support for an electrolytically grainable lithographic printing plate, which comprises carrying out twin-roll continuous casting and rolling and then hot rolling to prepare a strip coil having a thickness of 4 to 30 mm which is then heat-treated at a temperature of 300.degree. C. or above and cold-rolled with heat treatment at a temperature of 300.degree. C. or above being again carried out in the course of the cold rolling.
The above conventional processes are disadvantageous in that the production steps are complicated and involve time consuming treatment, inevitably increasing costs.
Further, in the above conventional processes, in order to attain a good electrolytically graining property and provide a good uniformity in appearance of the support after graining, conditions should be regulated for each of the steps of casting, heat treatment for homogenization, hot rolling, and intermediate annealing during cold rolling. In particular, in order to provide a good uniformity in appearance after graining, the regulation of grains should be carried out for each of the steps of casting, heat treatment for homogenization, hot rolling, and intermediate annealing during cold rolling.
Furthermore, steps requiring a high temperature and much time, such as heat treatment for homogenization and hot rolling, are required for the production of an aluminum alloy substrate having a desired thickness from a slab prepared by semicontinuous casting (DC casting). Even though each of the above steps can be successfully regulated, elements dissolved in supersaturation in a solid solution form during casting unfavorably precipitate during these steps conducted at a high temperature for a long period of time, resulting in the formation of coarse recrystallized grains during hot rolling. Even though subsequent heat treatment and working can provide small recrystallized grains, traces of the coarse recrystallized grains produced during the hot rolling remain as they are and appear as streaks (a streak pattern) extending in the rolling direction, which causes a lower uniformity in appearance of the electrolytically grained surface.
In the case of the processes disclosed in Japanese Unexamined Patent Publication (Kokai) Nos. 3-79798 and 5-156414 and the selection of improper conditions for the heat treatment in the course of cold rolling, the electrolytic graining is nonuniform, resulting in poor uniformity in appearance of the grained surface.
When an aluminum alloy substrate for a printing plate is electrolytically grained, it is a common practice to optionally carry out as a pretreatment chemical etching with an acid or alkali for degreasing or removal of oxide films from the surface of the substrate. The electrolytic graining process, as such, is an electrolytic etching process wherein an alternating current is applied using as a counter electrode graphite or the like to cause electrolytic etching, thereby forming pits on the surface of the substrate to provide a grained surface.
The above graining enhances an adhesion of a photosensitive film and water retention, beneficial to printing performance, to the printing plate. Since adhesion and water retention should be provided uniformly over the whole surface of the printing plate, pits should be formed uniformly over the whole printing plate. For a printing plate provided with a photosensitive film, the grained surface should have a uniform appearance when viewed with the naked eye because the results of development after the exposure and development are evaluated by visual inspection.
Nonuniform electrolytic graining means that proper surface roughness cannot be attained due to excessive etching (dissolution type) or the presence of a region remaining unetched in the electrolytic etching. In this case, a problem occurs associated with the suitability of the plate for use in printing. Specifically, the adhesion of a photosensitive film to the printing plate becomes poor, and, further, the water retention or corrosion resistance in nonimage areas deteriorates, which in turn leads to tinting or scumming in nonimage areas during printing.
Nonuniform appearance of the grained surface means nonuniform color tone such as observation of streaks (a streak pattern) along the rolling direction or partial loss of gloss to give a cloudy appearance. This is caused by nonuniform chemical etching as a pretreatment and electrolytic etching as an electrolytic graining treatment (nonuniform etching, the presence of a region remaining unetched or excessive etching) and an nonuniform metallic structure.
The nonuniform metallic structure is attributable to nonuniform grain orientation and grain size, coarsening and nonuniform dispersion of an intermetallic compound, and the like. Even when the nonuniformity of the metallic structure is of an extent that is not detrimental to the uniformity of electrolytic graining (including pretreatment) necessary for printing, it often makes the appearance of the grained surface remarkably nonuniform.
A nonuniform appearance, i.e., the presence of cloudy color shading, in the grained surface is very inconvenient to inspection of image areas after development. Specifically, the cloudy portions are present as they are in nonimage portions after development, and since they have a color tone similar to the image areas, it becomes difficult to visually judge whether or not the image areas can be satisfactorily developed.